The most important : an ICE is build in

It's important to note that the on-chip Debug gives you almost but not quite an ICE on chip.

In particular, the on-chip debug is unlikely to give you real-time trace, coverage, or timing analysis.
Some larger chips (eg ARM?) do have an on-chip debug trace buffer, but I don't think 8051s do (the Triscend E5 certainly doesn't)